1. Field of the Invention
The present invention generally relates to monolithic ceramic electronic components, methods for manufacturing the same, and electronic devices including the monolithic ceramic electronic components. More particularly, the present invention relates to an improvement in the structure of terminals of monolithic ceramic electronic components.
2. Description of the Related Art
A conventional type of monolithic ceramic electronic component, which relates to the present invention, is known as a “monolithic ceramic substrate”. The monolithic ceramic electronic component includes a composite body having a multilayered structure including a plurality of ceramic layers.
Inside the composite body, interconnecting conductors are provided to constitute a desired circuit by using passive elements such as capacitors and inductors. Outside the composite body, an active element such as a conductor IC chip and a portion of a passive element as required are mounted.
The resulting monolithic ceramic electronic component is mounted on a desired interconnection substrate and constitutes a desired electronic device.
The monolithic ceramic electronic component is used as an LCR composite high-frequency component for use in mobile communication terminal devices, and as a composite component combining an active element such as a semiconductor IC chip and a passive element such as a capacitor, an inductor, and a resistor, or simply as a semiconductor IC package for use in computers.
More particularly, the monolithic ceramic electronic component is widely used to constitute various kinds of electronic components such as module substrates, RF diode switches, filters, chip antennas, various package components, composite devices, etc.
FIG. 9 is a sectional view illustrating a conventional monolithic ceramic electronic component. A monolithic ceramic electronic component 1 shown in FIG. 9 includes a composite body 3 including a plurality of stacked ceramic layers 2. The composite body 3 is provided with interconnecting conductors each of which is located in association with a particular ceramic layer 2.
The interconnecting conductors are several first terminals 5 arranged on a first end surface 4 in the stacking direction of the composite body 3, several second terminals 7 arranged on a second end surface 6 opposite to the first end surface 4 of the composite body 3, several internal conductor layers 8 disposed at a particular interface between the ceramic layers 2, and several via-hole conductors 9 penetrating a specific ceramic layer 2.
The first terminal 5 is used for forming a connection with an interconnection substrate (not shown). In order to improve the bonding strength with the interconnection substrate, the first terminal 5 includes a conductor layer defined by a conductive paste that is applied by printing.
The second terminal 7 is used for forming a connection with a mounted component (not shown). In order to improve the bonding strength with the mounted component, as in the first terminal 5, the second terminal 7 includes a conductor layer defined by a conductive paste that is applied by printing.
FIGS. 10A to 10E show, in sequence, part of a typical method for manufacturing the monolithic ceramic electronic component 1 shown in FIG. 9. As shown in FIG. 10A, a ceramic green sheet 11, which will form the ceramic layer 2, is formed on a carrier film 10 of polyethylene terephthalate having a thickness of 50 μm to 100 μm. In this way, a composite sheet 12 in which the ceramic green sheet 11 is supported by the backing carrier film 10 is obtained.
During the subsequent steps, prior to a stacking step of the ceramic green sheet 11, the ceramic green sheet 11 is handled in the form of the composite sheet 12.
The reason for working the ceramic green sheet 11 with the carrier film 10 functioning as an undercoat is that the ceramic green sheet 11 has significantly low strength, is soft, and is breakable, and it is extremely difficult to handle the ceramic green sheet 11 by itself. The ceramic green sheet 11 in the form of the composite sheet 12 is easy to handle and to align during the process. Also, undesirable shrinking and undulation of the ceramic green sheet 11 can be prevented during the subsequent step of drying the conductive paste.
Next, as shown in FIG. 10B, several through holes 13 for forming the via-hole conductors 9 are formed in the composite sheet 12. Alternatively, the through holes 13 may be formed so as not to penetrate the carrier film 10 and may be formed only in the ceramic green sheet 11.
Next, as shown in FIG. 10C, by filling the through hole 13 with a conductive paste, a conductive paste section 14 which will be the via-hole conductor 9 is formed. At the same time, the conductive paste layer 15, which will be the internal conductor layer 8 or a second terminal 7, is formed by applying a conductive paste on the outer main surface of the ceramic green sheet 11. Subsequently, the conductive paste section 14 and the conductive paste layer 15 are dried.
Next, as shown in FIG. 10D, after the carrier film 10 is separated from the ceramic green sheet 11, a plurality of ceramic green sheets 11 are stacked so as to define a green composite body 16 which is the composite body 3 before firing.
The separation of the carrier film 10 may be performed prior to the stacking of the ceramic green sheet 11 as in the above description. The arrangement may be such that the ceramic green sheet 11 is stacked in the form of the composite sheet 12, having the surface provided with carrier film 10 facing upward, and the carrier film 10 is separated every time one of the ceramic green sheets 11 is stacked.
Next, as shown in FIG. 10E, a conductive paste layer 17, which will be the first terminal 5, is formed by applying a conductive paste on one end surface of the green composite 16 by printing. The conductive paste layer 17 is then dried.
It should be noted that the conductive paste layer 17, formed after the green composite 16 is obtained, may be used for the second terminal 7 and not for the first terminal 5. In such a case, the conductive paste layer for the first terminal 5 is provided by the conductive paste layer 15 formed by the step shown in FIG. 10C.
Next, the green composite 16 in the state shown in FIG. 10E is pressed in the stacking direction and is fired. Thus, the monolithic ceramic electronic component 1 shown in FIG. 9 is obtained.
The first terminal 5 and the second terminal 7 are plated with nickel and are then further plated with gold, tin, or solder, as required.
Although not shown in the drawings, the monolithic ceramic electronic component 1 is mounted on an interconnection substrate arranged to oppose the first end surface 4 so as to electrically connect via the conductive layer that constitutes the first terminal 5. A component is mounted on the second end surface 6 and is electrically connected with the conductive layer that constitutes the second terminal 7, but this is also not shown.
According to the manufacturing method of the monolithic ceramic electronic component 1 shown in FIG. 10, a step for applying the conductive paste by printing and a step for drying the same must be performed once again subsequent to obtaining the green composite body 16 in order to form the conductive paste layer 17 shown in FIG. 10E. Thus, there is a problem of reduced production efficiency due to these extra printing and drying steps.
It is also possible to use another process in which the conductive paste layer 17 is applied by printing, is dried, and is fired after firing the green composite body 16 in the state shown in FIG. 10D. In this case also, there is a problem of reduced production efficiency as in the above.
Since a screen printing technique is generally used in applying the conductive paste layer 17, reliability of the screen printing from the point of view of precision is not satisfactory. Accordingly, there is a problem of improper formation and displacement of the conductive paste layer 17, smudges in the patterns thereof, and irregularities in the thickness.
When a defective mother composite from which a plurality of the monolithic ceramic electronic components 1 are obtained, is used, all of the resulting monolithic ceramic electronic components 1 may be defective.
It should be noted that during the process in which the conductive paste layer 17 is formed after firing, it is possible to remove the conductive paste layer 17 and perform the printing step again when the above-described problems occur. It is, however, impossible to repair these defects in a process in which the conductive paste layer 17 is applied by printing prior to firing.
Furthermore, during the steps of pressing and firing the green composite body 16, the ceramic green sheet 11 and the ceramic layer 2 tends to be distorted in the direction of the main surfaces thereof. Accordingly, when printing is performed to form the conductive paste layer 17 on the mother composite, the conductive paste layer 17 may be misplaced due to the distortion.
After the step of pressing the green composite body 16, deflection may be found in the green composite body 16 or in the composite 3 after the firing. Thus, the surface on which the conductive paste layer 17 is applied by printing becomes irregular, resulting in the degraded precision of the printing.
Furthermore, the size of the components mounted on the second end surface 6 of the monolithic ceramic electronic component 1 is decreasing. For a mounted component provided with sheet-type terminal electrodes, such as a surface-mounted component, the plane size of each terminal electrode is now reduced to 0.6 mm×0.3 mm. For a mounted component provided with bump electrodes such as a semiconductor IC chip, the size of each bump electrode is reduced to, for example, approximately 70 μm in diameter, and the array pitch thereof is reduced to approximately 150 μm. Accordingly, the conductive layer used as the second terminal 7 must be reduced in size, but the screen printing technique is not capable of forming the conductive layer having such high precision.
Furthermore, an electronic component electrically connected by wire bonding, such as a semiconductor IC chip, is also used as the mounted component. In such a case, the diameter of the bonding wire is approximately 20 μm, and the width of a pad element required for wire bonding is approximately 80 μm. When the conductive layer formed by screen printing is used as the pad element, the cross-section of the thus formed conductive layer shows that there is a beveled part of approximately 20 μm to 30 μm wide at each edge due to surface tension of the conductive paste. Consequently, the flat portion of the pad element 80 μm in width becomes narrow, resulting in joining failure of the bonding wires.